This invention relates to syringe pumps.
Syringe pumps are used to supply medication to a patient from a pre-filled syringe via an infusion line. The syringe pump applies a force to the plunger of the syringe to drive medication into the infusion line at a controlled rate. The head of the plunger is retained in such a way as to allow the plunger to be pushed in but to prevent the plunger moving in of its own accord as a result of siphoning of fluid from the syringe barrel. The plunger head is usually retained by means of wedge-like arms that move across the forward surface of the head and force the rear surface of the plunger head against a forward facing surface on the plunger head retainer so as to clamp it firmly against this surface. Because the size and thickness of plunger heads vary in different syringes, the plunger head retainer may not be able to accommodate all the different heads, thereby restricting the range of syringes with which the pump can be used. If the plunger head retainer does not retain the plunger head securely, it may allow the plunger to move and allow fluid to siphon from the syringe.
It is common to measure the force exerted on the plunger head by the pump driver in order to be able to detect excess force, such as caused by an occlusion. It is an advantage to be able to respond to a low level of excess force without false alarms. The force sensor is, however, usually located relatively remotely of the plunger head so a margin must be allowed for friction and other forces acting on the drive train.
It is an object of the present invention to provide an alternative syringe pump and method of operation.
According to one aspect of the present invention there is provided a syringe pump adapted to receive a syringe of the kind having a plunger movable along a barrel, the pump including a plunger head retainer mechanism having a forwardly-facing surface adapted to engage a rear surface of the plunger head and to apply a forwardly-directed force to the plunger, and the plunger head retainer mechanism including force sensor means responsive to force exerted on the plunger head by the retainer mechanism during displacement of the plunger.
According to a second aspect of the present invention there is provided a syringe pump adapted to receive a syringe of the kind having a plunger movable along a barrel, the pump including a plunger head retainer mechanism including a pad member coupled with the plunger head mechanism via force sensor means, the pad member being adapted to engage a rear surface of the plunger head such that a forwardly-directed force is applied to the plunger via the pad member and such that the force sensor means is responsive to force exerted on the plunger head by the retainer mechanism during displacement of the plunger.
The retainer mechanism is preferably arranged initially to pull the plunger head rearwardly relative to the mechanism, thereby increasing force on the sensor, and is preferably arranged subsequently to relieve the force on the sensor.
According to a third aspect of the present invention there is provided a syringe pump adapted to receive a syringe of the kind having a plunger movable along a barrel, the pump including a plunger head retainer mechanism having a forwardly-facing surface adapted to engage a rear surface of the plunger head and apply a forwardly-directed force to the plunger, the retainer mechanism having at least one retainer member that is movable both laterally and axially so that it can be displaced laterally across the front surface of the plunger head and can be displaced axially rearwardly relative to the mechanism against the front surface of the plunger head.
The retainer mechanism preferably includes a force sensor coupled with the forwardly-facing surface, and the force sensor may be provided by a strain beam supporting a pad exposed on the forwardly-facing surface. Preferably the retainer mechanism includes a spring arranged to urge the pad forwardly. The retainer mechanism is preferably operable such that the or each retainer member is initially displaced axially rearwardly relative to the mechanism to apply force between the rear surface of the plunger head and the forwardly facing surface, the retainer member being subsequently displaced axially forwardly a small distance to reduce the force. The retainer mechanism preferably includes two retainer members located on laterally opposite sides of the plunger. Each retainer member may include an arm extending from a shaft, the shaft extending parallel with the axis of the syringe, such that the arm can be displaced laterally by rotating it about the shaft and such that the or each arm can be displaced axially by moving the shaft along its axis. Each shaft is preferably driven by a gear assembly including a rotatable, threaded shaft and a gear wheel threaded on the shaft such that rotation of the shaft initially moves the gear wheel along the shaft to the end of the shaft and thereafter continued rotation of the shaft rotates the gear wheel. Each shaft may carry a gear, the gear assembly and the gear on each shaft having cooperating flange and channel formations. The pump may include a position sensor arranged to detect a limit of movement of each retainer member.
According to a fourth aspect of the present invention there is provided a method of loading a syringe in a syringe pump comprising the steps of: locating a syringe on the pump with a plunger head retainer in a rear position and with a retainer member on the plunger head retainer in an open position; displacing the plunger head retainer forwardly until it contacts the rear surface of the plunger head; in response to contact with the plunger head, displacing the retainer member initially to a closed position forwardly of the forward surface of the plunger head; and thereafter displacing the retainer member to a rear position in contact with the forward surface of the forward surface of the plunger head.
According to a fifth aspect of the present invention there is provided a method of loading a syringe in a syringe pump comprising the steps of: displacing a plunger head retainer to a rear position; operating a motor in the plunger head retainer in one sense to move a retainer member radially outwardly and axially forwardly; locating a syringe on the pump; displacing the plunger head retainer forwardly until it contacts the rear surface of the head of a plunger of the syringe; rotating said motor in an opposite sense such that the retainer member moves initially to a closed position forwardly of the forward surface of the plunger head and thereafter to a rear position in contact with the forward surface of the forward surface of the plunger head.
According to a sixth aspect of the present invention there is provided a method of operating a syringe pump comprising the steps of: locating a syringe on the pump with a plunger head retainer in a rear position; displacing the plunger head retainer forwardly until it contacts the rear surface of the plunger head; retaining the plunger head with the plunger head retainer such as to prevent forward displacement of the plunger head relative to the retainer but to allow continued monitoring of force applied between the plunger head and the retainer; and advancing the retainer to move the plunger forwards and dispense contents of the syringe whilst monitoring the force between the plunger head retainer and the plunger head.